Synchronverters with fast current loops

Abstract

<p> Virtual synchronous machines (VSM) are inverters that behave towards the power grid like synchronous generators. Hence, they can be used as grid forming inverters and they can support the grid with inertia, droops, fault ride-through and more. Synchronverters are a much studied type of VSM. In this paper we present several improvements and innovations to the synchronverter control algorithm. We offer a complete design procedure for this algorithm. One change is meant to mitigate the fact that earlier designs are very sensitive to grid voltage measurement errors and processing delay, which may cause harmonic distortion and fluctuating amplitude of the grid-side currents. We propose to include a fast current controller as the internal control loop of the inverter. The design of this current controller involves delicate issues of compensating the delays and eliminating the DC components of the currents. The new design enables also a natural way for distortion-less current limitation. We present a smooth start?up procedure. Our simulations and experiments show that the current controller leads to a dramatic reduction of the sensitivity of the VSM to measurement errors, processing delay and grid voltage imbalance. </p>

Improvements to the VSM current controller for better grid performance

“…2. As part of the synchronverter control algorithm in [25], these current loops help to replace the actual filter impedance of the inverter with a virtual one. This means that the reference currents for the algorithm are calculated using the virtual impedance.…”

“…where e d and e q is a dq representation of the internal synchronous voltage, and v d and v q is a dq representation of the grid voltage (see [25]). The PWM generation is based on the control signals g, that express the desired output voltages from the inverter legs, averaged over one switching period.…”

“…We give tuning formulas and we have added a control loop for the zero component of the output currents. The new current loops are a part of a collection of proposed improvements to the basic synchronverter algorithm described in our new paper [25]. These changes were needed to address the problem of the excessive sensitivity of synchronverters to grid voltage measurement errors, converter imperfections and processing delays.…”

“…1. This paper serves as a preliminary discussion of only the fast current control loops and leaves the discussion of the other new features of the control algorithm to the new journal paper [25]. In [25], we propose additional new features, such as lead filtered frequency droop, virtual capacitor in the VSM, integration of the secondary control, and power limitations.…”

“…1. Control block diagram of a synchronverter, taken from [25]. The algorithm uses the virtual currents i virt to produce the electric torque T e , and also (after limitation) as references for the current control loops.…”